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+--- Thread: Mystery ailment (/showthread.php?tid=7938)
RE: Mystery ailment - John Cornforth - 19-08-2022
Hi
Some modern condensers are poorly made in that the electrical contact to the metallisation at each end of the metallised plastic "roll" inside the can is only by virtue of a slight squeezing during assembly. With time and temperature cycling the plastic roll gradually relaxes and without a spring washer or similar the contact pressure diminishes.
Given that condensers carry several amps of pulse current, it doesn't take much increase in internal resistance for problems to show up. This can sometimes appear only when hot, and after cooling everything comes good.
If you have a Lucas original that works - hang on to it !
RE: Mystery ailment - Nick Lettington - 19-08-2022
John has of course written an in-depth article about condensers on the DA7C website...
http://www.da7c.co.uk/technical_torque_articles/capacitors.htm
What I'm wondering, given the unreliability of the new ones, is if you can just use a 2 or 4 microfarad capacitor in their place?
RE: Mystery ailment - Bill Dixon - 19-08-2022
Nick, The correct value is 0.2microfarad.
RE: Mystery ailment - Nick Lettington - 19-08-2022
Yes... that... 0.2... what's a factor of 10 between friends! but the question remains..?
RE: Mystery ailment - Parazine - 20-08-2022
The RIFA/Kemet PME271 range of paper capacitors are recommended, certainly for magnetos. I'm not sure about whether the back emf value would be higher in a batter/coil system than in a magneto but it might be worth a try.
They're available in 100nF or 150nF, (0.1 or 0.15uF) from RS components and others.
RE: Mystery ailment - JonE - 21-08-2022
Are these things tiny enough to just rig one up externally in addition to the working condenser?
RE: Mystery ailment - JonE - 23-08-2022
https://uk.rs-online.com/web/p/film-capacitors/0210617 pic...
RE: Mystery ailment - John Mason - 23-08-2022
Everyone is going on about capacitors. Although this could be the cause of David’s erratic running or not running. Does anybody have an any further ideas such as coil. Faulty ignition switch or even doggy wiring. What baffles me is the recurring stopping after @ short time then being able to re start a little while later and on it goes.
John Mason.
RE: Mystery ailment - Graham Barker - 23-08-2022
I had a recurring fault with very similar symptoms. After multiple attempts to fix, finally with the help of David O, we appear to have sorted it.
The cause was a stripped thread in the distributor base for the screw that holds down the contact breaker base. It was the screw that goes in the slotted hole in the contact base. This screw is required to conduct electricity as well as locate the breaker. Because of the stripped thread, there was poor electrical contact with the threaded metal insert in the distributor base. One symptom of this was heating of the screw head a few minutes after switching on the ignition. It got so hot that it burnt my finger tip.
The poor connection was also confirmed by measuring a significant voltage drop.
Holding the child lead close to the cylinder head while flicking the contacts resulted in a weak spark provided that the lead was close to the head. When retracting the lead, the spark did not jump as big a gap as would be expected.
It's a long shot, but this possible cause may be worth checking.
Regards
Graham Barker
RE: Mystery ailment - MartinH - 24-08-2022
Not such a long shot. After similar problems in the past I have now added a wire between the distributor body and a suitable earthing point. Do this on all cars before troubleshooting mysterious ignition faults!